This invention relates to printers. More particularly, the invention relates to using a vacuum to reduce print medium cockle in printers.
Computer technology is continually advancing, expanding the need for computers in the personal, business, and academic fields. As the need for computers has grown, so too has the need for various peripheral devices for use with computers, such as printers. A wide variety of printers exist that operate in a wide range of manners, however all share the same fundamental purpose of generating a xe2x80x9chard copyxe2x80x9d of data, whether it be on paper, on transparencies, etc.
One type of printer, commonly referred to as an xe2x80x9cinkjetxe2x80x9d printer, operates by applying liquid ink directly onto a sheet of paper. An inkjet printer typically includes one or more cartridges, commonly referred to as xe2x80x9cpensxe2x80x9d, each having a print head formed with very small nozzles through which the ink drops are xe2x80x9cshotxe2x80x9d or xe2x80x9cfiredxe2x80x9d onto the paper. The particular ink ejection mechanism within the print head may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal print head technology. To print an image, the print head is scanned back and forth across a print zone above the sheet, with the pen shooting drops of ink as it moves.
Regardless of the type of print head technology used, when the ink is applied to the paper, the paper absorbs the moisture in the ink. During printing, the amount of moisture absorbed by a portion of the paper is dependent on a variety of factors, including the amount of ink applied to the portion (the more ink that is applied, the more moisture there is to absorb), as well as the composition of the ink (the more liquid there is in the ink, the more moisture there is to absorb).
When one or more portions of the paper absorb more moisture than other portions of the same sheet of paper, the different portions of the paper expand at different rates and in different amounts. This causes the paper to become wavy, wrinkled, or corrugated, an effect commonly referred to as xe2x80x9ccockle.xe2x80x9d Cockle is a problem on paper that has high concentrations of ink in some portions and no ink in other portions, such as a presentation slide that has a white border (which has no ink and does not expand) and an ink-saturated inner portion (which attempts to expand substantially). The outer border restricts the expansion of the inner portion and results in a significant degree of cockle. Cockle also becomes a greater problem as the thickness of the paper decreases (thicker paper is stiffer and better able to resist cockle growth). The rate at which ink is applied to the paper can also affect cockle growthxe2x80x94the slower the application of the ink the longer the time that one area of the paper is wet due to the ink having been applied while adjacent unprinted areas are dry.
The invention described below addresses these and other disadvantages of the prior art, using a vacuum to reduce cockle in printers.
In a printer, liquid ink is applied to a print medium as the medium is passed through the printer. A low pressure zone is generated along one surface of the print medium to hold a portion of the print medium substantially flat for a period of time during and after the liquid ink is applied to the print medium. By subjecting the portion of the print medium to the low pressure zone, cockling of the print medium is reduced.
According to one aspect of the invention, a porous belt and vacuum enclosure are used to generate the low pressure zone to keep the print medium substantially flat. When the print medium is fed into the print path of the printer, the medium is situated on the porous belt. The vacuum enclosure maintains the low pressure zone, pulling air through the porous belt to keep the paper substantially flat on the belt. Portions of the print medium remain on the porous belt and are subjected to the low pressure zone as the print medium is fed through the path for a period of time after ink is applied to the respective portion.